1. Field of the Invention
One application of the invention pertains to the field of porosimetry, or the measurement of the pore volume of substances. More particularly, the invention pertains to a method and apparatus for reducing bubble formation due to diffusion of gas through liquid.
2. Description of the Related Art
The prior art measures the pore volume of a sample in a liquid extrusion porosimeter using a weight and balance method. Specifically, as shown in FIG. 1, a sample (1) is wetted by a fluid and then placed above a membrane (2). A reservoir (8) of fluid (3) is located below the membrane (2). This fluid is the same type of fluid, which is used to wet the sample (1).
The sample (1) has larger pores (4) than the pores (5) of the membrane (2). Pressure (6) is applied, until liquid is forced out of the pores (4) in the sample, and into the reservoir (8) of fluid (3). The displaced fluid (7) flows out of the fluid reservoir (8) and is directed to a fluid displacement reservoir (9). The fluid displacement reservoir (9) is on a balance (10), which weighs the amount of the displaced fluid (7). A counterweight (11) on the balance (10) is used to determine the weight change due to the displaced fluid (7). This weight change is used in combination with calculations known in the art to determine the volume of the pores (4) in the sample (1).
The sealed system attempts to minimize air bubbles. However, the prior art preferably uses water or other high air diffusivity fluids. This increases the air bubbles in a sample, and potentially yields inaccurate results. Air bubbles also increase when the membrane (2) has small pores. The presence of air bubbles under the membrane (2) skews the measurement in the balance (10).
The apparatus of the present invention measures the porosity characteristics of a porous sample of material. The sample is preferably wetted, with the same type of fluid which is in the reservoir, prior to placing the sample on the wetted membrane in the porosimeter, or the fluid can be poured over the sample in the chamber. The wetting liquid spontaneously flows into the pores of the sample.
The porosimeter of the present invention comprises a source of pressure connected to a pressurizable chamber for holding the sample. The sample is supported by a membrane located between the sample and a fluid reservoir. The membrane has a plurality of pores with a size smaller than any of the sample pores, so that the bubble point pressure of the membrane is higher than the pressure needed to remove liquid from the pores of the sample. The reservoir and the membrane form a sealed chamber. The extruded fluid which passed from the pores of the sample through the pores of the membrane displaces the fluid in the fluid reservoir.
The fluid reservoir includes an inlet and an exit for fluid, such that in operation, as fluid enters the fluid reservoir through the membrane or the inlet, fluid already in the fluid reservoir is displaced through the exit. An inlet in the fluid displacement reservoir receives the fluid displaced from the fluid reservoir. A recirculation line receives fluid from the exit of the fluid displacement reservoir and circulates the fluid into the inlet of the fluid reservoir. The fluid moves through channels for efficient removal of gas bubbles. In a preferred embodiment, a pump is also included to recirculate the fluid. Fluid returned to the reservoir circulates over the bottom of the membrane, and sweeps air bubbles out of the reservoir. The removed air bubbles dissipate and do not affect the weight change measurements in the fluid displacement reservoir.